Project Summary: Respiratory surfaces are exposed to particles that are rapidly cleared in healthy lungs by two major non-inflammatory mechanisms: mucociliary clearance and phagocytosis. Particles trapped in mucus are removed by mucociliary clearance. Particles not removed in mucus are eliminated via phagocytosis by airspace macrophages (AMs). During homeostasis, resident AMs patrol the airspaces to detect or ingest particles and microbes during homeostasis without inducing inflammation. However, during an acute lung injury, monocyte-derived macrophages (MDAMs) are recruited to the lungs. Recruited MDAMs are more proinflammatory than the normally quiescent non-inflammatory resident AMs. Accordingly, MDAM presence in the lungs is transient and limited, and they undergo apoptosis and are cleared by phagocytosis during the resolution of inflammation. Siglec-F is a sialic acid (SA)-binding receptor that is frequently used as a common marker of AMs, with resident AMs expressing high cell surface levels of Siglec-F in the steady state. Recruited MDAMs are initially Siglec-F negative, and they upregulate Siglec-F in a manner that corresponds to their apoptosis during resolution. Interestingly, multiple lines of evidence support a receptor-ligand relationship between Siglec-F and the ?2,3-SA of Muc5b. Preliminary studies used high dose LPS to demonstrate that: 1) high Siglec-F levels are associated with MDAM apoptosis, 2) Siglec-F deficient mice have persistent MDAMs during resolution, and 3) ST3Gal3 deficient mice that lack the ?2,3-SA of Muc5b that provides and endogenous ligand of Siglec-F, are unable to survive high-dose LPS challenge. The central hypothesis of this proposal is that during innate lung inflammation recruited MDAMs upregulate the expression of Siglec-F whose stimulation mediates apoptosis of MDAMs and facilitates the resolution of innate lung inflammation. Two Specific Aims are proposed to test this hypothesis. Aim 1: To test the hypothesis that Siglec-F is required for the clearance of MDAMs by apoptosis and the resolution of inflammation. Aim 2: To test the hypothesis that MDAM apoptosis is mediated through extracellular engagement of the Siglec-F receptor by Muc5b ?2,3 sialosides. Completion of these Aims will identify mechanisms for the resolution of lung inflammation following acute lung injury, define a novel role for Siglec-F in MDAM function, and identify related avenues of investigation to explore in the future. The training plan employs a multi-disciplinary approach and a strong mentoring team in an ideally suited environment, thereby providing an excellent means for development of the applicant as an independent scientist.